slapd(8) provides the single-master replication semantics promised by the X.500/LDAP service model, which ensures the directory service manages concurrency of update operations. Use of multi-master replication, as provided in some directory vendors, may lead to permanent inconsistencies in directory information which may harm directory applications which expect the X.500/LDAP's single-master service model. See the Internet-Draft draft-zeilenga-ldup-harmful-xx.txt for a discussion in this area.
Kurt@OpenLDAP.org

OpenLDAP 2.4 has support for multi-master replication using syncrepl, which addresses many of the concerns Kurt raised. It requires reliable clock syncronization (e.g. NTP) between the participating servers.
quanah@openldap.org, hyc@openldap.org